feat: add ScyllaDB online store with vector search

[zseta]

Summary

Adds a first-class ScyllaDB online store to Feast, including support for vector similarity search via ScyllaDB's built-in vector<float, N> type and HNSW ANN index.

What's included

New online store

• feast/infra/online_stores/scylladb_online_store/scylladb.py
  • ScyllaDBOnlineStore implementing OnlineStore
  • ScyllaDBOnlineStoreConfig with all connection and tuning options
  • Full online_write_batch / online_read support
  • retrieve_online_documents_v2 for ANN vector search

Configuration

online_store:
  type: scylladb
  hosts: ["host_url1", "host_url2"]
  keyspace: feast_keyspace
  local_dc: AWS_US_EAST_1

Vector search

Tag any Field with vector_index: "true" and dimensions: "N" to enable ANN search:

Field(name="embedding", dtype=Array(Float32), tags={"vector_index": "true", "dimensions": "768"})

Then query with retrieve_online_documents_v2:

results = store.retrieve_online_documents_v2(
    feature_view=fv,
    requested_feature="embedding",
    query=[0.1, 0.2, ...],
    top_k=10,
)

Backed by a dedicated vector<float, N> column with a CUSTOM INDEX USING 'vector_index' (HNSW).

Changes

File	Change
sdk/python/feast/infra/online_stores/scylladb_online_store/scylladb.py	New — full implementation
sdk/python/feast/infra/online_stores/scylladb_online_store/__init__.py	New — package marker
sdk/python/feast/repo_config.py	Register "scylladb" short-name alias
pyproject.toml	Add scylladb optional extra with scylla-driver>=3.28.0,<4
docs/reference/online-stores/scylladb.md	New reference doc
docs/reference/alpha-vector-database.md	Add ScyllaDB to vector store matrix
docs/getting-started/genai.md	Mention ScyllaDB vector support
sdk/python/tests/integration/online_store/test_scylladb_online_store.py	Integration tests
sdk/python/tests/universal/feature_repos/universal/online_store/scylladb.py	Test store creator

Notes

• Uses scylla-driver
• Vector search requires ScyllaDB with the Vector Search feature enabled (available on ScyllaDB Cloud).
• Without vector-tagged features the store works as a regular CQL store against any ScyllaDB cluster.